Britain and its allies are "highly unlikely" to send troops to intervene if Russia invades Ukraine, Defence Secretary Ben Wallace told the Spectator magazine.

Britain and fellow G7 nations this week warned Moscow of "massive consequences" if it invades the former Soviet state but Prime Minister Boris Johnson said Friday that it would explore "diplomatic and economic" means to counter Russian "aggression".

"It is not a member of NATO so it is highly unlikely that anyone is going to send troops into Ukraine to challenge Russia," Wallace told the magazine in an interview this week.

"We shouldn't kid people we would. The Ukrainians are aware of that," he added.

Wallace said Britain could help Ukraine "with capacity building" adding that threats of "severe economic sanctions" were the most likely form of deterrent.

He floated the possibility of cutting Russia off from the Swift international payment network. 

The West says Moscow has readied some 100,000 troops near Ukraine, which has been battling a pro-Moscow insurgency in its east since 2014, but does not yet know whether Putin has made a decision on whether to invade.

"We aren't sure that he has made a decision but nevertheless his actions and his military preparations indicate that way... I think we should all be worried," said Wallace.